Ferrous metal tubes or pipes are commonly coated with a protective metal coating, such as zinc, referred to as galvanizing, aluminum, and alloys of zinc and aluminum, including Galvalume®, a trademark of BIEC International, Inc. In a typical application, a metal strip, referred to as a “skelp,” is first coated with a protective metal coating, the strip is then rolled into a tubular shape or an open seam tube and the opposed lateral edges of the open seam tube are then welded in a continuous process typically by heating the opposed lateral edges with an induction coil, melting the adjacent edges and the opposed lateral edges are then driven together or forged, forming a continuous welded seam. Where the strip is formed of steel, for example, the adjacent lateral edges of the open seam tube are heated to the melting temperature of steel, which is 2,300° F. or greater, whereas the melting temperature of the protective metal coating is substantially less. For example, a typical galvanized coating has a melting temperature of about 780° F. and aluminum has a melting temperature of about 1,200° F. Thus, the protective metal coating of the tube at and adjacent the welded seam will burn off or vaporize leaving the welded seam unprotected. Most precoated ferrous metal tubes or pipes fail by corrosion of the welded seam. Thus, an object of this invention is to provide a continuous method of applying an adherent protective metal coating to the internal and external surfaces of a welded seam of a metal tube and a method of coating the internal and external surfaces of a metal tube including the welded seam area with an adherent protective metal coating.
U.S. Pat. No. 6,042,659 assigned to the assignee of this application discloses a method of coating the external surface of a welded seam with an adherent protective metal coating, wherein a flux is first applied to the external surface of the welded seam, the seam area is then preheated, a protective metal coating is then applied over the exterior surface of the welded seam by thermal spraying, and the metal tube and the weld area is then heated in a full body induction coil, heating the welded seam to a temperature equal to or greater than the melting temperature of the protective metal coating, melting the protective metal coating over the external surface of the welded seam.
U.S. Pat. No. 6,290,786, also assigned to the assignee of this application, discloses a method of coating the internal surface of the welded seam of a metal tube by applying a paste of a particulate or powdered protective metal coating and a liquid flux over the internal surface of the welded seam, then preheating the welded seam to a temperature less than the melting temperature of the protective metal coating, followed by heating the metal tube and the welded seam with a fully body induction coil, thereby heating the protective metal coating on the interior surface of the welded seam to its melting temperature, melting the protective metal coating over the internal surface of the welded seam as described above. The metal tube is then quenched, freezing the protective metal coating and forming an adherent protective metal coating on the interior surface of the welded seam.
A problem with the methods of coating the welded seam of a metal tube as described in the above-referenced patents, particularly when used in combination, is that the exterior heating of the welded seam, as by induction heating, results in melting of the protective metal coating on the exterior surface of the welded seam before the protective metal coating on the interior surface reaches the melting temperature. The protective metal coating on the exterior surface of the welded seam then leaks or drips under gravity, which results in a roughened surface, and may result in insufficient coating on the exterior surface of the welded seam. The protective metal coating which drips from the exterior surface of the welded seam may also foul the induction heater and requires cleanup. Attempts have been made to reduce this problem by using a lower frequency induction heater to reduce the dwell or soak time and thus reduce dripping. The higher the frequency of the induction heater, the lower the penetration of the heat through the metal tube. For example, an induction heater having a frequency of 3,000 Hz results in heating the metal tube to a “depth” of 0.042 inches, whereas an induction heater having a frequency of 1,000 Hz heats the tube to a depth of 0.073 inches and an induction heater having a frequency of 500 Hz heats the metal tube to a depth of 0.10 inches. Thus, lowering the frequency of the induction heater would be desirable, particularly for metal tubes or pipes having a wall thickness of greater than the induction frequency penetration. Finally, external heating of the welded seam, including induction heating, results in spreading of the heat from the welded seam as the dwell time or soak increases, resulting in greater loss of the protective metal coating from the outer surface of the metal pipe adjacent the welded seam.
There has therefore been a long felt need to eliminate the problems associated with the methods of applying an adherent protective metal coating to the internal and external surfaces of a metal tube or pipe, particularly including the welded seam, which eliminates the problems set forth above.